If the process is centred precisely on the target mean, then the value of Cpk will be the same as the Cp. However, in practice the Cpk value calculated from the UTL will be different from that calculated from the LTL. In view of this, it is important to note that only the lower of the two values can be used to express capability.
In this example, you will see that the lower value is the one calculated from the LTL, and is 1.19. Don't forget to record this in the appropriate box on your calculation sheet:
Once again, as with the Cp value, it may be helpful to view this information graphically. The image below shows how the process mean and distribution relate to the upper and lower tolerance limits.
